Virtual ground array memory and programming method thereof

ABSTRACT

A method for programming a virtual ground array memory, which includes a first cell and a second cell adjacent to first cell, includes the following steps. First, the first cell is selected as a target cell, wherein the second cell has been programmed to have data. Next, the second cell is read and the data is recorded in a register. Then, the target cell is programmed. Next, a program verifying operation is performed on the second cell. Afterwards, the data recorded in the register is programmed back to the second cell when the program verifying operation performed on the second cell fails.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates in general to a virtual ground array memory and aprogramming method thereof, and more particularly to a virtual groundarray memory and a programming method thereof capable of solving anarray effect.

2. Description of the Related Art

In a conventional virtual ground array memory, adjacent two cells mayproduce an array effect due to different phases, thereby causing theshift of a threshold voltage and a read window loss or even the readfailure.

FIG. 1A (Prior Art) is a schematic illustration showing a conventionalvirtual ground array memory. Referring to FIG. 1A, a virtual groundarray memory 100 includes a first memory cell block 110 and a secondmemory cell block 120. Illustrations will be made by taking a boundarycell 111 in the first memory cell block 110 and a neighboring cell 121in the second memory cell block 120 as an example. After the neighboringcell 121 is programmed, a program verifying operation is performed onthe neighboring cell 121 in order to verify whether the thresholdvoltage of the neighboring cell 121 is sufficiently high.

FIG. 1B (Prior Art) shows a threshold voltage distribution of theneighboring cell 121. As shown in FIG. 1B, when the neighboring cell 121passes the program verifying operation, its threshold voltage is a highthreshold voltage 130. At this time, because a leakage current I_(leak)flowing from the neighboring cell 121 to the boundary cell 111 exists, asense current I_(sense) received by a sense amplifier (not shown) isunequal to a cell current I_(cell) of the neighboring cell 121.

Consequently, when the boundary cell 111 is being programmed, theleakage current I_(leak) decreases and the sense current I_(sense)increases because the threshold voltage of the boundary cell 111 issufficiently high. Thus, the neighboring cell 121 is judged as havingthe insufficiently high threshold voltage, and the threshold voltage ofthe neighboring cell 121 is shifted downwards from the high thresholdvoltage 130 to a high threshold voltage 131. Thus, the read window isreduced or even the read may fail in a more serious condition.

SUMMARY OF THE INVENTION

The invention is directed to a virtual ground array memory and a methodfor programming the virtual ground array memory, wherein a datarecovering algorithm is embedded in a programming procedure in order tosolve an array effect and eliminate a read window loss caused by thearray effect.

According to a first aspect of the present invention, a method forprogramming a virtual ground array memory is provided. The virtualground array memory includes a first cell and a second cell adjacent tothe first cell. The method includes the following steps. First, thefirst cell is selected as a target cell, wherein the second cell hasbeen programmed to have data. Next, the second cell is read, and thedata is recorded in a register. Then, the target cell is programmed.Next, a program verifying operation is performed on the second cell.Afterwards, the data recorded in the register is programmed back to thesecond cell when the program verifying operation performed on the secondcell fails.

According to a second aspect of the present invention, a virtual groundarray memory including a first memory cell block, a second memory cellblock and a register is provided. The first memory cell block has aboundary cell. The second memory cell block has a neighboring cell,which is adjacent to the boundary cell and has been programmed to havedata. The register records the data of the neighboring cell. After theboundary cell is programmed, a program verifying operation is performedon the neighboring cell. When the program verifying operation performedon the neighboring cell fails, the data recorded in the register isprogrammed back to neighboring cell.

The invention will become apparent from the following detaileddescription of the preferred but non-limiting embodiment. The followingdescription is made with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A (Prior Art) is a schematic illustration showing a conventionalvirtual ground array memory.

FIG. 1B (Prior Art) shows a threshold voltage distribution of aneighboring cell 121.

FIG. 2 is a flow chart showing a method for programming a virtual groundarray memory according to the preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides a virtual ground array memory and a programmingmethod thereof, in which a data recovering algorithm is embedded into aprogramming procedure in order to solve an array effect and eliminate aread window loss of a memory cell caused by the array effect.

FIG. 2 is a flow chart showing a method for programming a virtual groundarray memory according to the preferred embodiment of the invention.This method is applied to the virtual ground array memory, whichincludes a boundary cell and a neighboring cell adjacent to the boundarycell. The boundary cell is disposed in a first memory cell block and theneighboring cell is disposed in a second memory cell block. So, phasesof the boundary cell and the neighboring cell may be different from eachother. For example, the neighboring cell has been programmed to havedata, and the boundary cell is to be programmed. Because the boundarycell and the neighboring cell have different phases, the array effectmay be generated.

In the invention, first, in step 210, the boundary cell is selected as atarget cell, which is to be programmed. Meanwhile, the neighboring cellhas been programmed to have the data because it is disposed in thememory cell block having a different phase. Next, before the target cellis programmed, the neighboring cell is read and the data previouslystored in the neighboring cell is recorded in a register in step 220.This register is, without limitation to, a static random access memory(SRAM) in the virtual ground array memory, for example.

Then, in step 230, a programming operation is performed on the targetcell. In the substantial operating procedure of step 230, all cells inthe first memory cell block are sequentially programmed until the cellsare all programmed. After the target cell is programmed, the leakagecurrent flowing from the neighboring cell to the target cell isdecreased, the array effect is thus generated, and the read window lossis produced in the neighboring cell because the threshold voltage of thetarget cell is high enough.

Thus, in step 240, a program verifying operation is performed on theneighboring cell. Step 240 substantially judges whether the read windowloss of the neighboring cell caused by the array effect is too great. Ifthe neighboring cell has passed the program verifying operation, itrepresents that the read window loss of the neighboring cell is withinan acceptable range without influencing the correctness of reading thedata stored in the neighboring cell.

When the program verifying operation of the neighboring cell fails, itrepresents that the threshold voltage of the neighboring cell is shifteddownwards too much due to the array effect, thereby causing the toolarge read window loss and the read error. Thus, in step 250, the datarecorded in the register is programmed back to the neighboring cell.Consequently, a sense amplifier in the virtual ground array memorysenses that the data stored in the neighboring cell is still theoriginal data, and the read correctness may be ensured.

In addition, in the above-mentioned programming method of the virtualground array memory, if the data originally stored in the neighboringcell corresponds to a high threshold voltage (e.g., the data may be“0”), the failure of the program verifying operation performed on theneighboring cell represents that the high threshold voltage is shifteddownwards. If the high threshold voltage is shifted downwards too much,the read error may occur and the shifted threshold voltage may beregarded as a low threshold voltage. Thus, in step 250, the datarecorded in the register is programmed back to the neighboring cell.

In addition, if the data originally stored in the neighboring cellcorresponds to a low threshold voltage (e.g., the data is “1”), thefailure of the program verifying operation performed on the neighboringcell represents that the low threshold voltage is shifted downwards, andthe read error cannot occur. So, step 250 may be omitted.

The invention also discloses a virtual ground array memory having thecircuit architecture similar to that of the virtual ground array memory100 of FIG. 1A. The virtual ground array memory of the inventionincludes a first memory cell block, a second memory cell block and aregister. The first memory cell block has a boundary cell. The secondmemory cell block has a neighboring cell adjacent to the boundary celland has been programmed to have data. The register records the data. Theoperation principle of the virtual ground array memory has beendescribed in the above-mentioned programming method of the virtualground array memory, so detailed descriptions thereof will be omitted.

In the virtual ground array memory and the programming method thereofaccording to the embodiment of the invention, an embedded read windowrecovering algorithm is utilized when the boundary cell is beingprogrammed. Thus, the array effect generated on the boundary of thememory cell block in the virtual ground array memory may be eliminated,the read window loss of the neighboring cell caused by the array effectmay be eliminated, the correctness of reading the neighboring cell maybe ensured, and the read error of the neighboring cell may be avoided.

While the invention has been described by way of example and in terms ofa preferred embodiment, it is to be understood that the invention is notlimited thereto. On the contrary, it is intended to cover variousmodifications and similar arrangements and procedures, and the scope ofthe appended claims therefore should be accorded the broadestinterpretation so as to encompass all such modifications and similararrangements and procedures.

1. A method for programming a virtual ground array memory, whichcomprises a first cell and a second cell which is adjacent to the firstcell, the method comprising the steps of: selecting the first cell as atarget cell, wherein the second cell has been programmed to have data;reading the second cell and recording the data in a register;programming the target cell; performing a program verifying operation onthe second cell; and programming the data, which is recorded in theregister, back to the second cell when the program verifying operationperformed on the second cell fails.
 2. The method according to claim 1,wherein the data corresponds to a high threshold voltage, and the datais programmed back to the second cell when the program verifyingoperation performed on the second cell fails.
 3. The method according toclaim 1, wherein the first cell is disposed in a first memory cellblock, the second cell is disposed in a second memory cell block.
 4. Avirtual ground array memory, comprising: a first memory cell blockhaving a boundary cell; a second memory cell block having a neighboringcell, which is adjacent to the boundary cell and programmed to havedata; and a register for recording the data of the neighboring cell,wherein a program verifying operation is performed on the neighboringcell after the boundary cell is programmed, and the data recorded in theregister is programmed back to the neighboring cell when the programverifying operation performed on the neighboring cell fails.
 5. Thememory according to claim 4, wherein the data of the neighboring cellcorresponds to a high threshold voltage, and the data is programmed backto the neighboring cell when the program verifying operation performedon the neighboring cell fails.
 6. The memory according to claim 4,wherein the register is a static random access memory.